Android is meant to run with most the ram used up by apps. Closing apps you use a lot, or the system uses will hurt your battery, not help it. This also is what makes opening apps smooth and quick.

That said....if you're rooted, just use Greenify. It can hibernate any user apps you wish. So after use them, they will automatically be hibernated shortly after. I use it on every app I don't use often, especially games. I don't hibernate apps I use a lot throughout the day.

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